The present invention relates to a method and device for positioning a workpiece carrier.
With the object of DE 195 32 281, for instance, a workpiece carrier has become known which consists essentially of a base plate on which several traveling shafts are arranged, each connected immovably to a wheel, with at least one drive wheel provided that is driven by an electric motor. The known workpiece carrier can be displaced on a railway and, by means of wirelessly transferred information, it is capable of stopping at defined machining stations with the objective that defined machining processes can be performed on workpieces arranged on the base plate of the workpiece carrier.
From the cited publication, however, it is impossible to discern how the workpiece carrier in the rail system is to be positioned with high precision at a given machining station. This is important, however, for high-precision machining of workpieces fastened to the workpiece carrier. High machining precision is a requisite, particularly in machining and joining processes, with respect to the workpieces arranged on the workpiece carrier. The cited publication thus lacks a high-precision positioning of the workpiece carrier on the railway when the workpiece carrier stops at a machining station and the machining is to take place.
An aspect of the present invention involves a refined method and device for positioning a workpiece carrier in a machining station such that the workpiece carrier is positioned with high precision in the machining station.
An aspect of the present invention involves an apparatus for positioning a workpiece carrier in a machining station. The workpiece carrier is movable on a rail system and one or more machining stations are arranged along the rail system for machining a workpiece arranged on the workpiece carrier. The apparatus includes at least one positioning device arranged in an area of the machining station. The positioning device has fixtures for positioning and high-precision retention of the workpiece carrier with respect to a stationary reference surface. The positioning device further includes fixtures for decoupling the workpiece carrier from the rail system, and a deceleration and acceleration device for the workpiece carrier.
The essence of the invention is thus that the workpiece carrier is positioned, after entry into the machining station by means of at least one positioning device, with respect to a stationary reference surface, held in place with high precision and decoupled from the rail system and that, after machining of the workpiece, the workpiece carrier is released by the positioning device and can be moved further along the rail system.
With the technical teaching presented, there results the essential advantage that a high-precision positioning of the workpiece carrier in a machining station with respect to a stationary reference is achieved, with the workpiece carrier being decoupled from the rail system, so that the forces impinging from the machining and the mass forces of the workpiece carrier are absorbed exclusively by the positioning device. Thus, high-precision positioning with very good reproducibility is achieved.
This further results in the advantage that high-precision machining of workpieces arranged on the workpiece carrier is possible, because, according to a first preferred embodiment of the invention, the workpiece carrier does not remain in the rail system. The rail system may, after all, feature corresponding inaccuracies (for instance, depressions, curves, bumps or guidance imprecision) and these inaccuracies of the rail system could impair the machining precision. For this reason, the workpiece carrier is lifted off the rail system, pressed against a stationary reference of the positioning device and coupled to this reference surface by what amounts to a form fit.
With the technical teaching presented, there also exists the advantage that there is no more need to take into account an overall height that varies according to each of the different workpiece carriers because, independently of the overall height of the workpiece carrier, it is always raised sufficiently far off the rail system so that each workpiece carrier, independently of its overall height, comes to rest against a stationary upper reference surface, with respect to which the machining processes on the carrier plate of the workpiece carrier take place. Thus, different workpiece carriers of a widely varying construction can be positioned with equal precision in the positioning device, without the size and dimensions of the workpiece carrier mattering at all. There is therefore no need to take account of tolerances in the overall height of the workpiece carrier.
Neither is the positional precision of the positioning device of the invention dependent on the load weight of the workpiece carrier. That is, if the workpiece carrier remained on the rail system with different load weights and were positioned there, then there would exist the danger that the rail system might be pressed down or yield because of differing weights and thereby the machining precision could be impaired.
If, however, the workpiece carrier is lifted off the rail system according to the invention and pressed against a strong, highly loadable reference surface, then the machining of the workpiece arranged on the workpiece carrier is independent of the load weight of the workpiece carrier.
The invention is not restricted to a vertical lifting of the workpiece carrier off the rails of the rail system as above. In another configuration it is provided that the workpiece carrier remains on the rail system and that it is held tight laterally by appropriate form-fitting receptacles and couplings and thereby decoupled from the rail system in regard to active forces, so that a relieving of the rail system is achieved. In this second embodiment as well, the problem of high-precision positioning is solved with a separate positioning device. The latter-mentioned positioning device then acts essentially laterally on the workpiece carrier and fixes it in place on the rail system (without lifting it off the rail system), with the goal that different load weights of the workpiece carrier are no longer passed on to the rail system. These load weights are thus absorbed by the positioning device.
The invention is not limited either to standing workpiece carriers, i.e., workpiece carriers that roll on their running rollers on guide tracks of a rail system. The present invention pertains equally to so-called suspended systems, that is, those in which a workpiece carrier is arranged so as to move suspended on a monorail or multirail system (in the manner of the suspended train in Wuppertal).
In the embodiment of a workpiece carrier arranged so as to move suspended from the rail system as well, the two embodiments for the fixation of the workpiece carrier in the positioning device can come into consideration, namely, the raising of the entire workpiece carrier with the objective of de-suspending it from the track of the rail system and fixing it in place, on the one hand, and, on the other, the second embodiment, in which the workpiece carrier is only locked laterally to the positioning device such that corresponding load changes are no longer passed on to the rail system, but act instead on the positioning device and are absorbed by it.
In a refinement of the invention, it is preferred that a special deceleration and acceleration-device be associated with the positioning device. The aforesaid deceleration device ensures that the workpiece carrier, independently of its load weight, is always braked uniformly upon entry into the machining station. The braking behavior of this deceleration device is preferably independent of the load weight.
The aforesaid acceleration device has the task of releasing the workpiece carrier from the machining station or positioning device onto the rail system, wherein it receives a certain initial acceleration. The assignment of an initial acceleration to the workpiece carrier has the essential advantage that the electric motors arranged in the workpiece carrier do not have to tolerate any short-circuit current in accelerating from a stop to the prescribed travel velocity, but rather a certain initial motion is already assigned to the workpiece carrier by the aforesaid acceleration device and that a short-circuit current of the electric motors is therefore avoided. In this way, considerable energy for powering the electric motor or motors is saved, and the energy accumulation device arranged on the workpiece carrier can provide the workpiece carrier with a considerably longer service life.
From the acceleration device, therefore, a faster change of workpiece carriers from one workpiece carrier to the next workpiece carrier results. This turnaround time, that is, entry time into and exit time from the positioning device, influences the process rate of the entire workpiece carrier system on a rail system to a large degree overall. By using the acceleration device, it was possible for a reduction of the production rate by 10-20% to be achieved, precisely because the workpiece carriers are ejected from the positioning device at an accelerated rate.
The deceleration device for the workpiece carriers can feature various embodiments. In a preferred configuration, it is provided that this deceleration device is constructed as a slide that can be raised and lowered, accommodating, by means of a control surface, a positioning roller arranged on the workpiece carrier and converting horizontally directed travel motion into a deceleration motion.
Instead of such a mechanical deceleration device, however, other deceleration devices can also find application. A deceleration device of this type could, for instance, consist of a deceleration element which hydraulically or pneumatically counters the travel motion of the workpiece carrier, thus absorbing it and likewise braking it to zero velocity. In place of such a hydraulic or mechanical damping, an electromagnetic braking by means of an electromagnet can also be provided.
The same applies to the acceleration device to be described later. In a preferred configuration, a mechanical acceleration device which cooperates with the previously mentioned mechanical slide is preferred. Instead of such a mechanical acceleration device, however, hydraulic, pneumatic or electromagnetic acceleration devices may also find application. The same applies here as was previously said for the deceleration device, namely, that a hydraulically or pneumatically actuated acceleration cylinder can be used, it being preferred if the cylinder previously used for the deceleration motion is subsequently used for the provision of an acceleration motion.
In a third configuration, it can be provided that the workpiece carrier runs up against a free-blowing stream from a jet stream, the nozzle of which is controlled such that here, too, a gentle braking from the travel velocity to a standstill takes place and, further, this nozzle can also be used for the subsequent acceleration of the workpiece carrier out of the positioning device, the nozzle then blowing onto the workpiece carrier and accelerating it.
In another configuration, a mechanical energy accumulator can be used as an acceleration and deceleration device, first absorbing and storing the kinetic energy from the workpiece carrier as an energy accumulator. After completion of machining in the positioning device, this energy stored in the energy accumulator can be released and imparted to the workpiece carrier, which is then accelerated out onto the rail system by drawing upon this energy.